Mysqli Multi Query in OOPs PHP

Question

I don't Understand What is the problem in this code. everything is ok but query didn't execute with multi query. On the other hand every code is working but only this multi query not working

// database connection
$mysqli = new mysqli(DATABASE_HOST, DATABASE_USER, DATABASE_PASS, DATABASE_NAME);

    // insert invoice into database
$query = "INSERT INTO invoices (invoice,custom_email,invoice_date, invoice_due_date, subtotal, shipping, discount, vat, total,notes,invoice_type,status) 
    VALUES ('" . $invoice_number . "','" . $custom_email . "','" . $invoice_date . "','" . $invoice_due_date . "','" . $invoice_subtotal . "','" . $invoice_shipping . "','" . $invoice_discount . "','" . $invoice_vat . "','" . $invoice_total . "','" . $invoice_notes . "','" . $invoice_type . "','" . $invoice_status . "');";
    // insert customer details into database
    $query .= "INSERT INTO customers (invoice,name,email,address_1,address_2,town,county,postcode,phone,name_ship,address_1_ship,address_2_ship,town_ship,county_ship,postcode_ship) 
    VALUES ('" . $invoice_number . "','" . $customer_name . "','" . $customer_email . "','" . $customer_address_1 . "','" . $customer_address_2 . "','" . $customer_town . "','" . $customer_county . "','" . $customer_postcode . "','" . $customer_phone . "','" . $customer_name_ship . "','" . $customer_address_1_ship . "','" . $customer_address_2_ship . "','" . $customer_town_ship . "','" . $customer_county_ship . "','" . $customer_postcode_ship . "');";

    // invoice product items
    foreach ($_POST['invoice_product'] as $key => $value) {
        $item_product = $value;
        // $item_description = $_POST['invoice_product_desc'][$key];
        $item_qty = $_POST['invoice_product_qty'][$key];
        $item_price = $_POST['invoice_product_price'][$key];
        $item_discount = $_POST['invoice_product_discount'][$key];
        $item_subtotal = $_POST['invoice_product_sub'][$key];

        // insert invoice items into database
        $query .= "INSERT INTO invoice_items(invoice,product,qty,price,discount,subtotal) 
        VALUES ('" . $invoice_number . "','" . $item_product . "','" . $item_qty . "','" . $item_price . "','" . $item_discount . "','" . $item_subtotal . "');";
    }
    }

    header('Content-Type: application/json');


    // execute the query
    if($mysqli -> multi_query($query)){
        //if saving success
        echo json_encode(array(
            'status' => 'Success',
            'message' => 'Invoice has been created successfully!'
        ));

    } else {
        // if unable to create invoice
        echo json_encode(array(
            'status' => 'Error',
            'message' => 'There has been an error, please try again.'
            // debug
            //'message' => 'There has been an error, please try again.<pre>'.$mysqli->error.'</pre><pre>'.$query.'</pre>'
        ));
    

    //close database connection
    $mysqli->close();

}

so Please Help me with this code that how can i resolve this error and what is the error in this code.